PARIS (REUTERS) - Protesters attacked a police station and smashed bank windows during anti-labour reform rallies in France on Thursday while the hardline CGT union sought to choke off fuel supplies in a showdown with a government that said it would not back down.

Sixteen people were arrested in Paris where 18,000-19,000 marched according to police, far fewer than in the first protests against the labour law over the past three months.

In the southwestern city of Bordeaux, about 100 people targeted a police station, throwing objects and damaging a police car. In Paris and in the western city of Nantes, bank windows were broken and protesters clashed with police, who responded with tear gas.

The next big day of protests is planned on June 14, four days after the Euro 2016 soccer tournament opens in France.

The CGT warned it could be disrupted if the government refuses to withdraw the draft reform Bill.

"The government has the time to say 'let's stop the clock' and everything will be ok," CGT chief Philippe Martinez said when asked by Reuters if his union was willing to disrupt the soccer contest. The union wants the bill scrapped.

"We will disrupt the Euro (tournament) and the government will be the one to blame," 33-year-old sales worker and CGT representative Naima said during the protest march.

As turnout at protests has dwindled, the CGT has turned to sectoral strikes, with workers stopping work at oil refineries, nuclear power plants and the railways, as well as erecting road blocks and burning wooden pallets and tyres at key ports like Le Havre and near distribution hubs.

Prime Minister Manuel Valls said the government would not withdraw the law and would break up refinery blockades, saying there could be some tweaks to the reforms but not on any of its key planks. He was backed by the country's other big trade union, the CFDT.

"There is no question of changing tack, even if adjustments are always possible," said Valls, who rejected calls to scrap the part of the law that put the CGT on the warpath.

That section would let companies opt out of national obligations on labour protection if they adopt in-house deals on pay and conditions with the consent of a majority of employees.

The government pushed the bill through the lower house of parliament with a decree as it struggled to find enough lawmakers to back it. The final vote is expected in July.

Laurent Berger, head of the bigger CFDT union and a backer of the reform, said: "The political and industrial relations climate has turned hysterical... let's calm things down."

NO BACKING DOWN

The SNCF state train company said upwards of two-thirds of national, regional and local rail connections were operating, suggesting stoppages by railworkers were hurting less than last week when a similar strike halved the number of trains running.

After police intervention in recent days to lift blockades at refineries and fuel distribution depots, Valls said 20-30 percent of fuel stations were dry or short of certain fuels.

"The situation is less worrisome as of today," Transport Minister Alain Vidalies said. Deliveries of fuel from depots to the petrol pump were now improving, he said.

French nuclear power capacity was cut by as much as five gigawatts due to stoppages. That is equivalent to just over six percent of the country's total production capacity.

Even if power industry experts say the nuclear plant strike is unlikely to provoke major blackouts due to legal limits on strike action and power imports from abroad, the action usually raises running costs for the EDF power utility. Although France remained a net power exporter, its imports jumped on Thursday.

With dockers striking at the southern port of Marseille, the number of ships waiting at sea to offload oil, gas and chemicals rose to 21 from what would normally be about five, the port authority said.

Oil giant Total said all but one of its fuel distribution depots were working. It said, however, that two of its five refineries in France were at a standstill and two more set to halt in coming days.

Jean-Claude Mailly, leader of the smaller FO union that is also protesting, said as the Paris march began: "In football speak, it's time the prime minister took the red card back."